Optical communication is a technique that could play a central role in future broadband communications. Accordingly, for widespread use of the optical communication, the optical components used in optical communication systems are required to be higher in performance, smaller in size and lower in price. Optical communication devices using photonic crystals are one of the leading candidates for the next-generation optical communication components that satisfy the aforementioned requirements. Some of these devices have already been put into practical use, an example of which is a photonic crystal fiber for polarization dispersion compensation. Furthermore, recent efforts have had the practical goal of developing optical multiplexers/demultiplexers and other devices that can be used in wavelength division multiplexing.
A photonic crystal consists of a dielectric body in which a periodic structure is formed. Typically, the periodic structure is created by providing the dielectric body with a periodic arrangement of modified refractive index areas, i.e. the areas whose refractive index differs from that of the dielectric body. Within the crystal, the periodic structure creates a band structure with respect to the energy of light and thereby produces an energy region in which the light cannot be propagated. Such an energy region is called the “photonic band gap” or “PBG.”
Providing an appropriate defect in the photonic crystal creates a specific energy level within the PBG (“defect level”), and only such light that has a wavelength (or frequency) corresponding to the defect level is allowed to be present in the vicinity of the defect. A defect that is shaped like a point can be used as a resonator for the light having the aforementioned wavelength, whereas a linearly shaped defect can be used as a waveguide.
As an example of the previously described technique, Patent Document 1 discloses a two-dimensional photonic crystal having a body (or slab) provided with a periodic arrangement of modified refractive index areas, in which a linear defect of the periodic arrangement is created to form a waveguide and a point-like defect is created adjacent to the waveguide to form a resonator. This two-dimensional photonic crystal functions as the following two devices: a demultiplexer for extracting a component of light whose wavelength equals the resonance frequency of the resonator from the components of light having various wavelengths and propagated through the waveguide and for sending the extracted light to the outside; and a multiplexer for introducing the same light from the outside into the waveguide.
Including the one disclosed in Patent Document 1, many two-dimensional photonic crystals are designed so that a wide PBG is created for either a TE-polarized light, in which the electric field oscillates in the direction parallel to the body, or a TM-polarized light, in which the magnetic field oscillates in the direction parallel to the body. In this case, it is possible that no PBG is created for the other polarized light, or a PBG may be created for this polarized light but only under non-optimal conditions.
For example, if the photonic crystal is designed so that a PBG for the TE polarization (TE-PBG) is created and a defect level (resonance wavelength) due to a point-like defect (resonator) is created within the TE-PBG, it is possible that a PBG for the TM polarization (TM-PBG) is not created within the wavelength range of the TE-PBG. In this case, a TM-polarized light having the resonance wavelength does not resonate at the resonator. Therefore, in an attempt to demultiplex light having the resonance wavelength from broadband light passing through a waveguide located in the vicinity of the resonator, though the TE-polarized light can be almost completely extracted, the demultiplexing efficiency will be low since the TM-polarized light cannot be extracted. A similar problem also arises in the case of multiplexing.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-272555 (Paragraphs [0023] through [0027] and [0032]; FIGS. 1 and 5 through 6)